Sometimes it's because of price, if something's significantly cheaper on Amazon I'll buy it from Amazon. Free delivery tips it in Amazon's favour sometimes.

Sometimes it's availability. It can be quite painful to order things from outside the UK, most sellers aren't equipped to deal with VAT/duty their end so I end up with the delivery company holding packages hostage until I pay those (which is fine) plus a £20-30 "admin fee" for their package-hostage service. Pre-Brexit I could avoid this by ordering from elsewhere in the EU, but now I'm even more limited. Looking through my Amazon purchase history 3 of the last 5 things I've ordered are only available from Amazon in the UK (and at twice the price from a couple of dodgy looking online stores I've never heard of).

The other reason is their returns policy, if the thing I order doesn't work they'll take it back with no fuss and refund the money. Other major retailers in the UK make this difficult, frustrating and/or time-consuming.

I think they succeeded in retail because of a relentless focus on making the experience for customers better than their competitors. That's all still there, in the background - but it's being eclipsed by some other part of the org that wants to trick those customers into buying a load of shit.